Using electrospray ionization mass spectrometry (ESI-MS), we have demonstrated the direct observation of NO attached to the heme moiety in horse heart myoglobin (Mb) and in the a- and P- chains of human hemoglobin (Hb). It was found that a narrow range of EST-MS conditions conspire to make observation of Fe-NO interactions challenging, and this is presumably the reason why earlier attempts by other research groups to detect intact Fe-NO products by mass spectrometry were unsuccessful. For Mb and Hb, mass shifts are observed that are consistent with NO modification of the hemoproteins. ESI mass spectra of the apoprotein portions of Mb and Hb in the presence of NO demonstrated the absence of NO modification of the polypeptide backbones. To test the hypothesis that intact nitrosylated heme groups are observable by ESI-MS, a nitrosylated model metalloporphyrin was studied. To test further our hypothesis that the bemoprotein-NO peaks are due to heme nitrosylation and contain no significant contributions from NO modification of the polypeptide backbone, we determined the ESI-MS conditions necessary for observing S-nitrosation of Cys residues in Hb. Our findings demonstrate that (once correct conditions are established) ESI-MS is a powerful tool for the detection of intact Fe-NO interactions in proteins and porphyrins. A paper describing this work is in press in J. Am. Chem. Soc.